Correlated autoradiographic and ion-microscopic study of the role of iodine in the formation of “cold” follicles in young and old mice

Summary

The role of iodine in the formation of “cold” follicles (not labeled on autoradiograms after radioiodine administration) was analysed in ICR female mice during aging and involution of thyroid hyperplasia, by use of light and electron microscopy and by comparing autoradiographic and analytical ion-microscopic images for the same follicle in serial sections. The proportion of “cold” and “partly cold” (displaying a patchy or ring labeling pattern on autoradiograms) follicles increased significantly during aging. This increase was more pronounced in old mice fed an iodine-rich diet as compared to mice fed a moderate iodine diet. Similarly, during goiter involution produced by refeeding iodine, the follicular heterogeneity of iodine metabolism was more accentuated with a high dose of iodine, regardless of the age of the mice. The follicular lumina of “hot” and “cold” follicles had the same concentration of stable iodine, as shown by analytical ion microscopy, and the cells of both types of follicles formed colloid droplets in response to TSH. Furthermore, when a goitrogenic treatment was induced in aged mice, some “cold” follicles persisted after 8 days, but all follicles resumed “hot” after 16 days. By analytical ion microscopy, ¹²⁷iodine was also found inside thyroid cells of old mice, but the cytoplasmic patches of ¹²⁷iodine were not labeled with ¹²⁵iodine. They corresponded to lipofuscin pigments and secondary lysosomes, as observed in serial sections at the electron-microscopic level. This intracellular stable iodine could constitute a slow turnover compartment not used for hormone synthesis.